The present invention relates to evaluation and measurement of an optical waveform employed for high-speed optical fiber transmission.
In the existing optical fiber transmission to transmit information, optical signals are modulated according to intensity thereof into binary values, i.e., on and off. The optical signal modulation speed is rapidly increasing from 2.5 gigabits per second (Gbits/s) to 10 Gbits/s and then to 40 Gbits/s. The transmission distance of the optical fiber transmission is restricted by the wavelength dispersion of the optical fiber used for the optical transmission. It is known that the distance theoretically lessens in inverse proportion to the square of the bit rate. At a bit rate of 2.5 Gbits/s, the maximum transmission distance of the optical fiber transmission is about 1000 kilometers (km). The value rapidly lowers to about 60 km at a bit rate of 10 Gbits/s and only 4 km or less at a bit rate of 40 Gbits/s. Therefore, maintaining and elongating the transmission distance are quite a serious issue in the high-speed optical fiber communication. Particularly, a phenomenon called “frequency chirp” in an optical modulator to modulate optical signals is a phenomenon in which undesired phase modulation is added to the optical signal in association with the optical signal modulation conducted according to the intensity thereof. It is known that the optical signal transmission distance varies up to several times the original distance according to presence/absence and magnitude thereof. Therefore, evaluation of the frequency chirp characteristic is quite important for the optical modulator and the optical signals. The characteristic is precisely evaluated by a measuring device, for example, a chirp measuring device described in “Q7607 Optical Chirp Test Set” of ADVANTEST.
The amount of information items (transmission capacity) transmissible through one optical fiber cable is successively increasing due to growth in the number of wavelengths employed for the wavelength multiplexing and increase in the modulation speed. However, the total transmission capacity is limited to about 10 terabits per second (Tbits/s) and is remaining at this level for several years. This is because the wavelength band available for the optical transmission has reached the limit which is restricted by the wavelength band (corresponding to about 80 nanometer=10 THz for C, L, and S bands) of the optical fiber amplifier. It is hence not possible to increase the number of wavelengths and hence the transmission capacity cannot be expanded only by improving the signal bit rate as described above. Resultantly, it is assumed as essential that a new modulation method is developed to expand the transmission capacity to improve the utilization ratio of the frequency band so that the frequency band includes as many optical signals as possible.
To meet the requirement for the transmission efficiency improvement and the transmission distance elongation, there has been recently proposed a Phase Shift Keying (PSK) method in which the intensity as well as the phase of optical signals are modulated for the information transmission. For example, a Quadrature PSK (QPSK) method described in “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration” written by R. A. Griffin, et. al. (OFC2003, paper PD-FD6, 2003) is a method of modulating optical signals in which with the amplitude of electric field (intensity) of optical signals kept unchanged, the phase of the electric field is modulated into four values, i.e., 0°, 90°, 180°, and 270°. When compared with the conventional binary intensity modulation to transmit one-bit information by use of one symbol, this method employing the four-value phase state enables to transmit information of two bits by use of one symbol. To transmit the same amount of information items under the same condition, the modulation speed (symbol rate) can be lowered to one half of the original value. It is therefore possible that the frequency utilization efficiency is improved by transmitting the same amount of information items using about one half of the frequency band. At the same time, the transmission distance can be elongated by reducing the influence from the wavelength dispersion. As transmission methods positively employing the phase modulation, there have been proposed, for example, an optical duo-binary modulation, a Carrier Suppressed RZ (Return-to-Zero) (CSRZ) modulation, and an Amplitude and Phase Shift Keying modulation to modulate the phase and the amplitude at the same time.
In the modulation method employing the optical signal phase, the information is transmitted using a plurality of mutually different phase points (constellation) obtained by synthesizing the electric filed amplitude and the phase angle of optical signals each other. Therefore, it is important to precisely modulate the amplitude and the phase of optical signals according to a high-speed electric digital signal. Also in the conventional binary modulation, to conduct high-bit-rate or high-speed modulation at a bit rate equal to or more than gigabits per second, it is required to detect the reflection of a high-speed electric signal inputted to the optical modulator and the inter-symbol interference due to, for example, the insufficient frequency band of parts and integrated circuits (ICs) of the modulator to thereby possibly suppress deterioration in the optical waveform. To meet the requirement, precise waveform observing methods such as a method using an eye line display and a method using a histogram display have been explored to be broadly employed (reference is to be made to, for example, “Agilent 86100C Infiniium DCA-J” of Agilent Technologies. Especially, in the multivalue modulation to transmit information using a plurality of phase points, specifically, three or more phase points, the eye opening is larger than the binary signal. Also, transition takes place between the plural phase points to cause complicated inter-symbol interference. This leads to a problem of the difference between points of timing of the plural electric modulation signals. Therefore, it is required to even more precisely observe the waveform. The complicated variations in the phase and the intensity cause further complicated waveform variations after the transmission due to the wavelength dispersion and the nonlinear effect of the optical fiber and the detection in the receiver. To predict and to improve the transmission characteristics, it is quite important to precisely evaluate the optical electric field waveform of optical signals outputted from the optical modulator.
However, the phase component cannot be directly detected by, for example, a photodiode. It is difficult to observe the phase component of optical signals and the direct measurement of the optical electric field waveform of optical signals is rarely carried out. Examples of observation of phase points in multivalue phase modulation signals have been reported in “Direct measurement of constellation diagrams of optical sources” written by C. Dorrer, J. Leuthold and C. R. Doerr in post-deadline paper PDP-33, OFC, 2004 and “Unrepeated 210-km Transmission with Coherent Detection and Digital Signal Processing of 20-Gb/s QPSK Signal” written by Dany-Sebastien Ly-Gagnon, Kazuhiro Katoh and Kazuro Kikuchi in paper OTuL4, OFC/NFOEC 2005, 2005. For example, the former reports the direct measurement of phase points of the QPSK signal for the first time. Description will now be given of two phase measuring methods of the prior art.
The frequency chirp measuring apparatus represented by Q7607 of Advantest as described in “Q7607 Optical Chirp Test Set” of ADVANTEST is a measuring apparatus using an optical frequency discriminator such as a Mach-Zehnder interferometer to convert the optical frequency chirp (frequency variation) into optical intensity in the observation.
According to “Direct Measurement of Constellation Diagrams of Optical Sources” and “Unrepeated 210-km Transmission with Coherent Detection and Digital Signal Processing of 20-Gb/s QPSK Signal” described above, digital phase diversity measurement is employed to observe allocation of phase points of multivalue phase modulated optical signals. This is a kind of the coherent homodyne detection method in which there is prepared a local optical source substantially equal in the wavelength to the signal light to cause homodyne interference between local light emitted from the local optical source and the signal light to receive the resultant light by a photodiode. The optical electric component of the signal light is converted into intensity of the electric signal. This signal is sampled at a high speed by an oscilloscope in a real time fashion and is converted into a digital data string to be then transferred to a personal computer. After having received the data string, the computer analyzes the data of a predetermined period of time to estimate and to calculate allocation of phase points of the phase modulation signal. Description will next be given in detail of the principle of the operation by referring to FIG. 8.
A pulse pattern generator 112 generates binary pseudo-random electric digital signals (D1, D2) of two systems of, for example, 10 Gbits/s. The signals are amplified respectively by driver circuits 106-1 and 106-2 to be inputted via drive signal input lines 107-1 and 107-2 to an optical modulator 108 as a measurement object. A laser beam emitted from a laser source 101 is modulated by these symbol patterns to be converted into phase modulation light such as optical QPSK signals of 20 Gbits/s. This signal as an observation object is fed to a signal light input port 132 of an optical phase diversity circuit 113. To a reference light input port 135, local light emitted from a local light laser source 140 is inputted, the source 140 continuously oscillating (or producing short pulses) with an optical frequency almost equal to that of the input light.
The optical phase diversity circuit 113 is an optical circuit also called “optical 90-degree hybrid” which divides each of the local light and the signal light which are inputted thereto into two portions to output the divided signals to two output ports 133 and 134, the signals having substantially the same intensity. In the configuration, the in-phase component output port 133 is adjusted such that the local light interferes with the signal light when their optical phases are in an in-phase state. The quadrature component output port 134 is adjusted such that the local light interferes with the signal light when their optical phases are in an quadrature state. Light from each port is fed to a high-speed optical oscilloscope 141 to be converted into an electric signal. From the signal received from the in-phase output port 133, there is obtained an optical electric field waveform of the component which is in phase with the reference light of the signal light. From the signal received from the quadrature output port 134, there is obtained an optical electric field waveform of the component which is orthogonal to the reference light of the signal light.
The high-speed oscilloscope 141 operates at a sampling speed of 10 to 20 giga-samples per second to conduct high-speed analog-to-digital (A/D) conversion for each of the in-phase and quadrature electric field waveforms at a speed of one to two samples per bit (at a central time of eye opening in an ordinary case) as shown in FIG. 9A. The resultant data is sequentially stored in an internal memory (about 100000 bits in an ordinary case). When the measurement is continuously conducted, the memory is full of data in about several microseconds. The measurement is once stopped and all data 121-2 is transferred in a batch from the memory to a Central Processing Unit (CPU) 122. In the CPU 122, the data items are mapped in a two-dimensional graph in which the in-phase (I) component and the quadrature (Q) component of the same time are associated respectively with the ordinate and the abscissa of the graph. This re-constructs the optical electric field phase, and a signal 121-3 indicating the resultant graph is outputted to a display 123.
Actually, each of the local light and the signal light includes large phase noise in a band from several hundred kilohertz to several megahertz, and it is difficult to completely equalize the two lasers with each other in the emission wavelength. Therefore, the optical phase of each laser beam is unstable and includes deviation at a high speed of the order of several megahertz to about one hundred megahertz. This results in a problem that the phase angle φ of the phase point in FIG. 9B rotates at a high speed of several megahertz to about one hundred megahertz and hence the state of the phase of the optical signal cannot be measured. For this difficulty, according to “Direct Measurement of Constellation Diagrams of Optical Sources” described above, there is executed processing in which on assumption that the optical phase at a point at which the intensity of optical pulses is fixed is observed and the optical phase rotates at a fixed speed by the phase deviation, the phase deviation component is estimated to be removed. In “Unrepeated 210-km Transmission with Coherent Detection and Digital Signal Processing of 20-Gb/s QPSK Signal” described above, there is executed digital data processing which mathematically calculates the fourth power of the synthesized electric component to remove the phase modulation component in 90-degree units due to the phase modulation. From the remaining fractional components of the phase, a phase rotation component from several megahertz to about one hundred megahertz is detected to be removed. According to the report of “Direct Measurement of Constellation Diagrams of Optical Sources” above, the allocation of the phase points of the QPSK signal are directly measured for the first time as a result.